1. Field of the Invention
The present invention is related to a shaft coupling, particularly to a shaft coupling for transmitting the drive from a drive shaft to a driven shaft, both the shafts being split coaxially. Furthermore, the present invention is related to a device for driving a function unit of an image forming device, particularly to a device for driving a function unit of an image forming device such as a copying machine, a printer, a facsimile, and a multifunction device that uses the shaft coupling to transmit the drive of a motor to a driven device such as a photoconductor.
2. Background Information
Conventionally, in an image forming device such as a copying machine employing an electrostatic copying system, an image forming unit transfers a toner image to a recording medium, and the recording medium to which the toner image is transferred is sent along a conveyance path to a fixing unit. In the fixing unit, the toner image is fixed to the recording medium, and then the recording medium to which the toner image is fixed is discharged to a copy receiving tray.
Recently, the image forming unit of the above image forming device includes a photoconductor unit. The photoconductor unit includes a photoconductor having a surface on which an electrostatic latent image is formed, and a developing device for supplying the photoconductor with the toner to develop the electrostatic latent image on the surface of the photoconductor into a visible toner image. The photoconductor unit is detachably attached to a main body of the image forming unit, so that it is easy to replace the photoconductor units and to perform a jam-clearing process near the image forming unit when a paper jam occurs.
In some image forming devices, a shaft coupling is provided in order to connect a revolving shaft (driven shaft) of the photoconductor and a drive shaft of the motor located in the image forming unit main body. In this device, when the photoconductor unit is to be attached to a predetermined position in the image forming unit main body, a front cover of the image forming unit main body is opened, the photoconductor unit is slid from the front side to the rear side of the image forming unit main body, and the revolving shaft of the photoconductor is connected to the drive shaft of the motor in the image forming unit main body via the shaft coupling.
FIG. 18 shows a shaft coupling 100 used in the conventional image forming device. In FIG. 18, the shaft coupling 100 consists of a drive male coupling 102 attached to a drive shaft 101, a female coupling 103 spline-engaged with the drive male coupling 102, and a driven male coupling 105 fixed to a driven shaft 104 which is coaxial with and slidable relative to the drive shaft 101, the driven male coupling 105 capable of being engaged with or disengaged from the female coupling 103. The shaft coupling 100 shown in FIG. 18 connects the driven shaft 104 (a revolving shaft as a power transmission shaft) of the driven device (for example, a photoconductive drum, a developing device, and the like) with the drive shaft 101 at the motor side such that both the shafts rotate integrally (refer to Japanese Patent Application Publication 2001-200858).
However, in the conventional shaft coupling 100 as shown in FIG. 18, the engagement portion between the female coupling 103 and the drive male coupling 102, and the engagement portion between the female coupling 103 and the driven male coupling 105, are spline engagements. Therefore, the axial length thereof will lengthen, and the size of the entire structure will become enlarged, thus making it difficult to use the shaft coupling 100 in a machine or device which must be as small as possible, such as an image forming device.
In order to solve the above-mentioned problem, a structure may be proposed as shown in FIG. 17. A shaft coupling 59 shown in FIG. 17 includes a drive coupling 62 and a driven coupling 60, the drive coupling 62 being formed with a concave portion 63 (inner circumference recess) on a face opposing to a driven coupling 60 and the driven coupling 60 being formed with a convex portion 61 (inner circumference projection) on a face opposing to the drive coupling 62. The drive coupling 62 and the driven coupling 60 are urged against each other by means of the urging force of a spring (not shown in the drawings), so that the convex portion 61 of the driven coupling 60 is engaged with the concave portion 63 of the drive coupling 62. As a result, the short sliding travel of the driven coupling 60 enables the concave portion 63 of the drive coupling 62 and the convex portion 61 of the driven coupling 60 to be engaged with each other and to be disengaged from each other.
In the shaft coupling 59 shown in FIG. 17, it is necessary to keep a circumferential clearance (wc) at the engagement portion between the concave portion 63 and the convex portion 61 in order for the concave portion 63 of the drive coupling 62 and the convex portion 61 of the driven coupling 60 to be smoothly engaged with each other. Therefore, if the torque of the driven device is large and the fluctuation of the torque is large, the concave portion 63 and a tip of the convex portion 61 may slide against each other and thereby cause wear. In addition, if the center axes of the drive shaft and the driven shaft are slightly misaligned, the wear on the concave portion 63 and the convex portion 61 may become extreme, due to a small amount of sliding at the convex-concave engagement between the drive coupling 62 and the driven coupling 60 which generates a phenomenon known as “coupling skip”. “Coupling skip” is a phenomenon in which the driven coupling 60 compresses the spring to slide away from the drive coupling 62 with a small distance in order to disengage the convex portion 61 from the concave portion 63, and after the drive coupling 62 and the driven coupling 60 slip relative to each other, the concave portion 63 and the convex portion 61 are engaged again with each other by the urging force of the spring, thereby causing the engagement position of the concave portion 63 and the convex portion 61 to shift in the circumferential direction.
In view of the above, there exists a need for a shaft coupling and an image forming device having the same which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.